Process for manufacturing succinic acid

ABSTRACT

A process for the preparation of succinic acid comprising the steps of:
     a) providing an aqueous medium comprising magnesium succinate by fermentation of a carbohydrate source, in the presence of a magnesium base;   b) processing the aqueous medium wherein the magnesium succinate is treated with a monovalent base, prior to or after a crystallization step, to provide a magnesium base and an aqueous solution comprising a monovalent succinate salt;   c) adjusting the concentration of the monovalent succinate salt to between 10 and 35 wt. %;   d) subjecting the aqueous solution to water-splitting electrodialysis, to produce a first solution comprising monovalent base and a second solution comprising succinic acid and monovalent succinate salt, the electrodialysis causing conversion of 40 to 95 mole %;   e) separating the second solution into succinic acid and the monovalent succinate salt by crystallization;   f) recycling the monovalent succinate salt solution of step e) to step d).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 National Stage Application ofInternational Application No. PCT/EP2011/052128, filed Feb. 14, 2011 andpublished as WO 2011/098598 on Aug. 18, 2011, in English, which in turnis based on and claims benefit of U.S. Provisional Application No.61/303,767, filed Feb. 12, 2010.

BACKGROUND

The discussion below is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter.

An aspect of the present invention pertains to a process formanufacturing succinic acid in high purity in an economical manner.

Succinic acid is often manufactured via fermentation of carbohydrates bymicro-organisms. A common feature to all fermentation processes is theneed to neutralise the acids excreted by the micro-organisms. A drop inpH below a critical value, depending on the micro-organism used in theprocess, could damage the microorganism's metabolic process and bringthe fermentation process to a stop. Therefore, it is common practice toadd a base in the fermentation media in order to control the pH. Thisresults in the succinic acid produced being present in the fermentationmedia in the form of a succinate salt.

Despite the longstanding practice to produce succinic acid viafermentation, one of the challenges in the manufacture of succinic acidis still to obtain the acid in a relatively pure form while at the sametime carrying out the process in an economical manner on a scale whichis commercially attractive.

Electrodialysis is one of the purification processes that may be used inthe production of succinic acid via fermentation. Water-splittingelectrodialysis in particular allows the direct conversion of thesuccinate salt into succinic acid and base. In this type ofelectrodialysis bipolar membranes are generally used to split water intoH⁺ and OH⁻ respectively, which combine with the anion and cation of thesuccinate salt respectively, resulting in the production of separatesolutions of succinic acid and base.

There is still need for a process for manufacturing succinic acid whichprovides succinic acid in high purity and which can be performed in aneconomical manner with a low power consumption, without producingsubstantial amounts of non-reusable components (i.e. waste by-products)and without substantial yield loss.

SUMMARY

This Summary and the Abstract herein are provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary and the Abstract are notintended to identify key features or essential features of the claimedsubject matter, nor are they intended to be used as an aid indetermining the scope of the claimed subject matter. The claimed subjectmatter is not limited to implementations that solve any or alldisadvantages noted in the Background. An aspect of the presentdisclosure includes a process where magnesium succinate is provided byfermentation and treated by means of crystallisation and salt exchangeto provide an aqueous solution of a monovalent succinate salt which isespecially suited for subsequent water-splitting electrodialysis.Succinic acid of high purity is produced by using water-splittingelectrodialysis with a partial conversion of the succinate salt tosuccinic acid, separating the succinic acid from the succinate salt bycrystallisation and recycling the succinate salt to the electrodialysisprocess.

DETAILED DESCRIPTION

It has been found that the process for the manufacture of succinic acidas described herein is very efficient and economical, provides highproduction yields, minimal product losses and results in succinic acidof high quality.

Accordingly, an aspect of the present invention pertains to a processfor the preparation of succinic acid comprising the steps of:

a) providing an aqueous medium comprising magnesium succinate byfermentation, wherein a carbohydrate source is fermented by means of amicroorganism to form succinic acid, a magnesium base being added asneutralising agent during fermentation to provide the magnesiumsuccinate;b) subjecting the aqueous medium comprising magnesium succinate to acrystallisation step and a salt exchange step to provide an aqueoussolution comprising a monovalent succinate salt, wherein the saltexchange, which is performed either prior to or after crystallisation,comprises treating the magnesium succinate with a monovalent base toprovide a magnesium base and the monovalent succinate salt;c) adjusting the concentration of the monovalent succinate salt in theaqueous solution to a value between 10 and 35 wt. %;d) subjecting the aqueous solution comprising the monovalent succinatesalt to water-splitting electrodialysis, to produce a first solutioncomprising monovalent base and a second solution comprising succinicacid and monovalent succinate salt, the electrodialysis being carriedout to a partial conversion of 40 to 95 mole %;e) separating the second solution comprising succinic acid andmonovalent succinate salt into succinic acid and a solution comprisingthe monovalent succinate salt by crystallisation;f) recycling the solution of step e) comprising the monovalent succinatesalt to step d).

The use of magnesium base during the fermentation step a) advantageouslyresults in the formation of magnesium succinate, which is soluble in thefermentation broth. The inventors have found that a separate andcontrolled crystallisation can be performed from a fermentation brothneutralised with magnesium base. This is not the case when using otherbases such as a calcium base. The use of calcium base generates calciumsuccinate, which tends to crystallise during fermentation in a lesscontrolled manner than magnesium succinate. In addition, the calciumsuccinate crystals obtained tend to be more difficult to separate fromthe fermentation broth.

As a result of the crystallisation and salt exchange steps performed onthe magnesium succinate obtained via fermentation, the aqueous solutioncomprising monovalent succinate salt provided in step b) is of suchquality that it may be directly subjected to water-splittingelectrodialysis to provide succinic acid.

Carrying out the water-splitting electrodialysis to a partial conversionof 40 to 95 mole % and subsequently recycling the remaining succinatesalt to the electrodialysis step advantageously results in an optimalprocess with low power consumption and no substantial yield loss.

Furthermore, the process as described herein produces virtually no wasteby-products, since all compounds formed and separated in the differentsteps may be recycled. The magnesium base of step b) may for instance beused in the fermentation step a) and the solution comprising monovalentbase of step d) may be used in the salt exchange of step b). Theseparation step e) also contributes to minimise the amount ofnon-reusable components since it does not generate further wasteby-products.

The aqueous medium comprising magnesium succinate is provided by afermentation process. The magnesium succinate salt is generally alreadypresent in an aqueous medium when it leaves the fermentation. In such aprocess, a carbohydrate source is fermented to succinic acid by means ofa succinic acid-producing micro-organism. During fermentation, amagnesium base is added as neutralising agent. This results in theformation of an aqueous medium comprising the corresponding magnesiumsuccinate salt.

The base anion of the magnesium base is preferably chosen from at leastone of hydroxide, carbonate and hydrogencarbonate, and more preferablyis hydroxide. Although the use of magnesium as the base cation ispreferred, another alkaline earth metal cation, such as a calciumcation, may also be used. The amount of alkaline earth metal base addedis determined by the amount of succinic acid produced and may bedetermined via pH control of the fermentation medium.

The biomass (i.e. microbial cell matter) may be removed from thefermentation broth before further processing of the succinate-containingmedium. Biomass removal may be effected, for example, by conventionalmethods including filtration, flotation, sedimentation, centrifugation,flocculation and combinations thereof. It is within the skills of theskilled person to determine an appropriate method. Other optionaltreatments prior to further processing include washing, filtration,(re)crystallisation, concentration and combinations thereof.

The aqueous medium comprising the alkaline earth metal succinate salt,preferably magnesium succinate, is subjected to a crystallisation stepand a salt exchange step to provide an aqueous solution comprising amonovalent succinate salt. The monovalent succinate salt obtained isespecially suitable for water-splitting electrodialysis since it issubstantially free of fermentation-derived products (e.g. sugar,protein, amino acids) which may negatively interfere in water-splittingelectrodialysis by, for instance, increasing the power consumption andfouling of the ion-permeable membranes.

The salt exchange step, which may be performed either prior to or aftercrystallisation, comprises treating the alkaline earth metal succinatesalt with a monovalent base to provide an alkaline earth metal base andthe monovalent succinate salt.

The monovalent base used in the salt exchange is preferably a hydroxide,carbonate and/or hydrogencarbonate, more preferably a hydroxide, of amonovalent cation, the monovalent cation being sodium, potassium,lithium, ammonium, monoalkylammonium, dialkylammonium, trialkylammoniumor tetraalkylammonium, preferably sodium or potassium and morepreferably sodium. Generally, the use of sodium and potassium basesadvantageously results in a higher conversion of the alkaline metalearth succinate salt to the monovalent succinate salt than when ammoniumbases are used. This is relevant for preparing a product with a lowalkaline earth metal ion content suitable for water-splittingelectrodialysis. The base anion is generally chosen to correspond to thebase anion used as neutralising agent during fermentation.

The amount of monovalent base is determined by stoichiometric and pHconsiderations. It may be preferred to use a surplus of base to obtain ahigh conversion and to ensure the removal of virtually all alkalineearth metal ions from the succinate.

The alkaline earth metal base obtained as a result of the salt exchangeof step b) may be recycled to the fermentation step a).

The crystallisation may comprise at least one of a concentration step,such as a water evaporation step, a cooling step, a seeding step, aseparation step, a washing step and a re-crystallisation step.Concentration may be performed as a separate step or together withcrystallisation (e.g. evaporative-crystallisation).

When crystallisation is performed prior to salt exchange, the alkalineearth metal succinate salt is crystallised from the aqueous mediumprovided by fermentation by concentrating the fermentation broth (e.g.by evaporation of water), preferably after biomass removal. The alkalineearth metal succinate crystals obtained are then separated from theliquid phase, which contains the fermentation-derived products,providing a purified alkaline earth metal succinate salt. The saltexchange may then be performed in batch or in continuous mode. In batchmode, an aqueous solution comprising a monovalent base is slowly addedto a solution or slurry containing the alkaline earth metal succinatesalt. The alkaline earth metal base formed in the salt exchange steptypically is in solid form while the monovalent succinate salt isdissolved in the aqueous phase. The salt exchange may preferably beperformed in continuous mode. When the salt exchange is performed incontinuous mode, a slurry of the alkaline earth metal succinate saltcrystals (e.g. magnesium succinate) and an aqueous solution of themonovalent base (e.g. sodium hydroxide) are mixed in a reactor togenerate a slurry comprising the alkaline earth metal base in solid form(e.g. magnesium hydroxide) and the monovalent succinate salt dissolvedin the aqueous phase (e.g. sodium succinate). The two resultingcomponents may be separated by conventional solid-liquid separationprocesses, such as filtration and/or sedimentation, providing theaqueous solution comprising the monovalent succinate salt.

When the salt exchange is performed prior to crystallisation, themonovalent base is added to the aqueous medium comprising the alkalineearth metal succinate salt provided by fermentation, preferably afterbiomass removal. As discussed above, the solid alkaline earth metal baseformed may be separated from the aqueous medium comprising the aqueoussoluble monovalent succinate salt. The monovalent succinate salt is thencrystallised from the aqueous medium by concentrating the aqueous medium(e.g. by evaporation of water) and the crystals are separated from theliquid phase, which contains the fermentation-derived products,providing a purified monovalent succinate salt. The aqueous solutioncomprising the monovalent succinate salt may be obtained by for exampledissolving the separated succinate crystals in water.

The aqueous solution of monovalent succinate salt may be subjected toadditional treatments prior to water-splitting electrodialysis, such asion exchange treatment, activated carbon treatment, desaltingelectrodialysis, dilution, concentration and/or filtration (e.g.nanofiltration). For instance, as a safety measure to prevent a too highalkaline earth metal level in the aqueous solution comprising themonovalent succinate salt, an ion exchange step may be performed priorto electrodialysis to lower the alkaline earth metal content thereof.

However, the process as described herein advantageously does notnecessitate such additional treatments, especially when a magnesium baseis added in the fermentation process to provide a magnesium succinatefermentation broth.

The aqueous solution comprising the monovalent succinate salt is thensubjected to water-splitting electrodialysis.

The initial concentration of the monovalent succinate salt in theaqueous solution that is subjected to electrodialysis (the feedsolution) is between 10 and 35 wt. %. Preferably, the monovalentsuccinate salt concentration is between 20 and 35 wt. %, more preferablybetween 20 and 30 wt. % and most preferably between 22 and 28 wt. %.Depending on the salt concentration, the aqueous solution comprising themonovalent succinate salt may be used directly after step b), or, ifnecessary, may be diluted or concentrated to adjust the saltconcentration prior to water-splitting electrodialysis. Concentrationmay be carried out by for instance evaporation or desaltingelectrodialysis.

The concentration of the monovalent succinate salt in the aqueous mediummay be determined by methods known to the skilled person, for instanceby using conductivity measurements or Inductively Coupled Plasma massspectrometry analysis.

The water-splitting electrodialysis is carried out to a partialconversion of 40 to 95 mole %. Preferably, the electrodialysis iscarried out to a conversion of 50 to 95 mole %, more preferably of 60 to95 mole %, even more preferably of 70 to 90 mole %, even more preferablyof 80 to 90 mole %, and most preferably of 85 mole %. A first solutioncomprising monovalent base and a second solution comprising succinicacid and monovalent succinate salt are produced in this process.

A partial conversion of 40 to 95 mole % means that 40 to 95 mole % ofthe monovalent succinate salt is converted into succinic acid. Thisresults in the second solution produced by the electrodialysiscomprising succinic acid in an amount of 40 to 95 mole %, calculated onthe total molar amount of succinic acid and succinate present in thesolution.

The degree of conversion may be monitored by measuring conductivity ofthe second solution using methods known to the person skilled in theart.

In addition to the conversion level and the initial salt concentrationof the feed solution, the conductivity of the second solution willdepend on the temperature of the electrodialysis process. The higher thetemperature at which the electrodialysis is performed, the lower thepower consumption will be. Hence, the working temperature is chosen tooptimise power consumption without compromising the performance and thelife of the ion-specific permeable membranes. Generally, thewater-splitting electrodialysis is performed at a temperature between25° C. and 40° C. However, it is preferred to conduct theelectrodialysis at a temperature higher than 50° C., for instancebetween 60° C. and 80° C., to allow for a low power consumption and thepossibility for heat recovery.

Because of the limited solubility of succinic acid in water, in order toavoid crystallisation of succinic acid during water-splittingelectrodialysis, the working conditions of the electrodialysis arechosen to ensure that the concentration of succinic acid in the finalsolution is below saturation. For instance, for a conversion of 40 to 95mole % and a working temperature of 25° C., at which the solubility ofsuccinic acid in water is about 8 wt. %, the initial sodium succinateconcentration should be between 10 and 25 wt. %. When working at highertemperatures, the concentration of sodium succinate in the feed solutionmay be higher.

The water-splitting electrodialysis as described herein may be performedusing a conventional apparatus and conventional methods. Preferably thewater-splitting electrodialysis is carried out in an electrodialysisapparatus provided with a cation exchange membrane and a bipolarmembrane. A typical water-splitting electrodialysis cell comprises aseries of a two compartment unit, generally a series of about 50 units.The aqueous medium comprising the monovalent succinate salt isintroduced in the salt/acid compartment (or feed compartment). Themonovalent cations are transported from the salt/acid compartment to thebase compartment through the cation exchange membrane to produce thefirst solution comprising the monovalent base. Simultaneously, H⁺ ionsare transported to the salt/acid compartment to produce the secondsolution comprising succinic acid and monovalent succinate salt.

It is preferred to apply the water-splitting electrodialysis tomonovalent succinate salts of sodium and potassium. When using ammoniumsuccinate, care must be taken to control the emission of toxic ammoniaresulting from the generation of ammonium hydroxide.

The second solution produced by the water-splitting electrodialysis isseparated into succinic acid and a solution comprising the monovalentsuccinate salt by crystallisation.

The succinic acid may be crystallised in a static crystallisation unit,by fractional crystallisation, by suspension crystallisation and/or bywash column crystallisation. The crystallisation may comprise aconcentration step, such as a water evaporation step, a cooling stepand/or a seeding step and one or more washing steps. The crystals maythen be separated from the liquid phase of the solution crystals byfiltration or centrifugation.

The solution containing the monovalent succinate salt obtained afterseparation step e), which may comprise residual succinic acid, isrecycled to the water-splitting electrodialysis. This recycling stepensures that no substantial yield loss is suffered as a consequence ofthe partial conversion of the succinate into succinic acid duringwater-splitting electrodialysis.

The succinic acid obtained after the separation step e) is generally insolid form (e.g. crystalline) and has a purity of at least 99 wt. %,preferably at least 99.5 wt. %, more preferably at least 99.7 wt. % andmost preferably at least 99.9 wt. %.

The succinic acid obtained by the process according to an aspect of theinvention is of high purity and is suitable for direct use in numerousapplications such as synthetic processes, food applications and cosmeticapplications. The succinic acid can be directly used as a monomer inpolymerisation processes (e.g. for the formation of polyamides) or as aprecursor of other important products and synthetic intermediates suchas succinic acid esters, succinic acid anhydride and diamino butane. Thesuccinic acid obtained is particularly suited for the production ofbutanediol (e.g. by hydrogenation), which is an important intermediaryproduct in polymer production.

The process as described herein advantageously is accompanied by a lowpower consumption and ensures that no or substantially no wasteby-products are generated.

Aspects of the present invention are further illustrated by thefollowing Examples, without being limited thereto or thereby.

Example 1 Crystallisation of Magnesium Succinate

In a jacketed 0.5 L vessel 150.0 g of magnesium succinate tetrahydrate(synthesised from succinic acid 99% from Acros and magnesium oxide 98%from Acros) was suspended in 199.9 g of demineralised water, in order toobtain a magnesium succinate content of 28 wt. % (expressed asanhydrate). To this mixture 8.1 g of sodium lactate (60%, Purasal S fromPurac), as well as 2.6 g of sodium acetate (anhydrous from Fluka) and10.0 g of yeast extract paste (65% from Bio Springer) were added tosimulate a succinic acid fermentation broth and to track the presence ofimpurities in the final magnesium succinate crystals.

The mixture was heated by means of a thermostatic bath to 90° C., inorder to dissolve all solids. After 30 minutes the solution stillcontained solids. Each 30 minutes some water was added to a total amountof 147.4 g. At this stage all de solids were dissolved and the totalvolume was about 450 ml.

The mixture was cooled from 90 to 20° C. in 5 hours and allowed to stirduring the night. No solids were formed. The mixture was heated again to80° C. and 100 ml of water was allowed to evaporate.

Then the concentrated solution was cooled from 80° C. to 60° C. in 30minutes and seed crystals were added. Then the mixture was cooledlinearly from 60° C. to 20° C. in 3 hours. During the coolingcrystallization nucleation took place at 37° C.

The resulting suspension was separated by means of a filteringcentrifuge. After centrifugation an amount of 72.8 g of solid magnesiumsuccinate was obtained.

The samples were analysed on sodium content, lactate and acetatecontent, total nitrogen content and colour (APHA, a known method for themeasurement of colour). The results are shown in Table 1.

TABLE 1 acetic lactic total fresh acid acid Na nitrogen colour sample(wt. %) (wt. %) (mg/kg) (mg/kg) (APHA) 1 Crystals 0.22 0.23 820 390 12¹2 Mother liquor 0.69 0.68 6400 3100 n.a. ¹This is the colour of a 10%solution in water at 50° C.

The amount of impurities in the magnesium succinate crystals issignificantly reduced compared to the amount of impurities in the motherliquor. In addition, the colour-value measured in the solution of themagnesium succinate crystals indicates that the residual colour in thecrystals is very low.

The purity of the magnesium succinate crystals may be improved bywashing of the crystals.

Example 2 Salt Exchange of Calcium Succinate and Magnesium Succinatewith Monovalent Base

Preparation of Starting Materials:

For preparation of magnesium succinate in an aqueous medium (solution),80.0 g of succinic acid were dissolved in 1000.0 g of water. Afterheating to 50° C., a stoichiometric amount of solid magnesium oxide(27.3 g) was added. To make sure all of the succinic acid would react, asmall surplus (2.3 g) of MgO was added. Finally, the mixture wasfiltered over a Büchner funnel, equipped with a filter paper. Thefiltrate, being a 9.4 wt. % solution of magnesium succinate, wascollected. Calcium succinate in an aqueous medium (suspension) wasprepared in an analogous manner by letting succinic acid (80.0 g+4.2 gsurplus in 1000.1 g water) react with solid calcium hydroxide (50.6 g).After filtration and washing with approximately 800 ml of demineralisedwater, the residue (calcium succinate) was collected and dried in adesiccation stove for 18 hours at 80° C. The calcium succinate was thensuspended in water. The slight surplus of reagents in both reactions wasapplied in order to obtain succinates with a minimal amount ofimpurities.

Experiments:

Magnesium succinate and calcium succinate were reacted with variousbases to investigate the effectivity of the salt exchange process. Thefollowing monovalent bases were used: sodium hydroxide [NaOH], sodiumcarbonate [Na₂CO₃], ammonium carbonate [(NH₄)₂CO₃] and ammoniumhydroxide [NH₄OH].

The reactions were carried out in 500 ml beakers or Erlenmeyer flaskscontaining 100 ml of 10 wt. % magnesium succinate or calcium succinatein aqueous medium. Sodium carbonate and ammonium carbonate were added insolid form in stoichiometric amounts. Ammonia and NaOH were added insolute form, also in stoichiometric amounts. The reaction mixtures werestirred using a stirring bar and a magnetic stirrer. The amounts ofalkaline earth metal succinate and monovalent base used in each reactionare shown in Table 2.

TABLE 2 m(Mg/Ca-Succ.) Base Exp. # Reaction [g] [g] 1 MgSucc + NH₄OH99.7 9.2 2 MgSucc + NaOH 100.0  10.8 (+89.4 H₂O) 3 MgSucc + Na₂CO₃ 99.97.2 4 MgSucc + (NH₄)₂CO₃ 99.6 6.5 5 CaSucc + NH₄OH 10.0 + 89.8 H₂O 8.8 6CaSucc + NaOH 10.0 + 90.1 H₂O 10.5 7 CaSucc + Na₂CO₃ 10.0 + 90.1 H₂O 6.88 CaSucc + (NH₄)₂CO₃ 10.0 + 90.1 H₂O 6.1

The mixtures were allowed to react for 1 hour. From each reactionmixture, samples of 25 ml were taken. These were centrifuged, afterwhich Mg (or Ca) and succinate were determined analytically. Theanalytical data and the initial concentration of Mg²⁺/Ca²⁺ or succinatewere used for calculation of the conversion of magnesium succinate orcalcium succinate to sodium or ammonium succinate. The results are givenin Table 3.

TABLE 3 Mg/Ca Succinate Conversion Experiment pH [ppm] [wt. %] [%] 1:MgSucc + NH₄OH 9.6 8415 6.9 43.0 2: MgSucc + NaOH 12.4 12 4.0 99.8 3:MgSucc + Na₂CO₃ 10.5 880 7.7 94.1 4: MgSucc + (NH₄)₂CO₃ 7.8 9487 7.537.4 5: CaSucc + NH₄OH 11.1 3489 1.0 12.7 6: CaSucc + NaOH 13.0 281 6.595.4 7: CaSucc + Na₂CO₃ 10.5 19 7.0 99.3 8: CaSucc + (NH₄)₂CO₃ 8.0 8297.0 98.7

As can be seen from Table 3, when sodium hydroxide is used, a conversionof well above 90% is obtained both for magnesium succinate and forcalcium succinate. The same applies when sodium carbonate is used. Forammonium carbonate it should be noted that while for calcium succinate aconversion of 98.7% is obtained, the conversion for magnesium succinateis only 37.4%.

Example 3 Partial Electrodialysis of a Sodium Succinate Solution

An Electrocell electrodialysis module (Sweden) was equipped with aFumatech FBM bipolar membrane, and a Neosepta CMB cation exchangemembrane. A set-up with two electrode compartment and one feedcompartment was used. The membrane areas of the bipolar and the cationexchange membrane was 0.01 m². The first compartment comprised of theanode and the cation exchange side of the bipolar membrane, the secondfeed compartment of the anion exchange side of the bipolar membrane andthe cation exchange membrane, and the third compartment of the cationexchange membrane and the cathode. 2 wt. % sulfuric acid in water wascirculated through the anode compartment to ensure a high conductivity.A 30.5 wt. % sodium succinate solution was circulated through the middlecompartment as a feed. The feed solution was prepared by dissolving237.6 g sodium succinate in 540.8 g demineralised water. A 6 wt. %sodium hydroxide solution was circulated through the cathode compartmentto ensure a high conductivity at the cathode side, and to collect thesodium hydroxide produced. The three solutions were circulated with aperistaltic pump at 250 ml/min from a 500 ml glass buffer over theelectrodialysis module. The glass buffer vessels were double walled. Thesulphuric acid, sodium hydroxide were reagent grade, and the Puracsodium succinate was of high purity food grade quality.

The temperature across the three compartments was kept between 40 and60° C. with a water bath. The electrodialysis experiment was carried outa constant 7.5 A DC current. No crystallisation of succinic acid wasobserved in the electrodyalisis module during the experiment.

During water-splitting electrodialysis the sodium succinate solution inthe feed compartment of the module is acidified batch wise throughsodium removal through the cation exchange membrane to form sodiumhydroxide in the cathode compartment, while protons generated by thebipolar membrane form succinic acid with the original succinate ions.

In the beginning of the experiment the conductivity of the feed (sodiumsuccinate solution) was about 160 mS/cm and the voltage about 10 V.During the first 250 minutes of the experiment the voltage increasedslowly to 11 V, coinciding with a conductivity decrease. In the intervalbetween 550 and 626 minutes the voltage had increased from about 12 V to16 V and the conductivity decreased from about 50 mS/cm to 14.62 mS/cm.At this point the conversion was 95% and the experiment was stopped.

Voltage increase results in a rapid increase in power consumption toconvert the residual sodium succinate.

The solution comprising 36.7 wt. % of succinic acid and 2.8 wt. % ofsodium succinate was cooled down to room temperature, during whichsuccinic acid crystals formed. The fluid was then poured off and thesolid succinic acid was dried in a stove for 15 hours.

Example 4 Crystallisation of Succinic Acid from a Succinic Acid/SodiumSuccinate Solution

In a crystalliser (500 ml open jacketed glass vessel) 100.1 g (0.848mole) of succinic acid (Acros) and 19.9 g (0.074 mole) of sodiumsuccinate hexahydrate (Acros) were dissolved in 281.5 g of demineralisedwater, by heating with the thermostatic bath to 80° C. This resulted ina clear solution with 25% of succinic acid and 3% of sodium succinate,representing a solution obtained from a water-splitting electrodialysisprocess with a conversion of 92 mole %. The solution was cooled from 80°C. to 20° C. in 5 hours with a linear cooling profile. During coolingnucleation took place between 56° C. and 50° C.

The resulting crystals were separated by means of a filteringcentrifuge. After centrifugation an amount of 70.2 g of solid succinicacid was obtained.

Samples of the motherliquor and the succinic acid crystals were analysedon sodium content, while the motherliquor was also analysed on succinatecontent. The crystals were analysed without drying.

The sodium content of the crystals was found to be 165 ppm, as comparedto 10400 ppm in the mother liquor, whereas the content of succinic acidin the mother liquor was of 12 wt. %.

The amount of sodium was considerably reduced in the crystals whencompared to the motherliquor. The amount of sodium in the crystals maybe lowered further by washing during centrifugation.

The invention claimed is:
 1. A process for the preparation of succinicacid comprising: a) providing an aqueous medium comprising magnesiumsuccinate by fermentation, wherein a carbohydrate source is fermented bymeans of a micro-organism to form succinic acid, a magnesium base beingadded as neutralising agent during fermentation to provide the magnesiumsuccinate; b) subjecting the aqueous medium comprising magnesiumsuccinate to a crystallisation step and a salt exchange step to providean aqueous solution comprising a monovalent succinate salt, wherein thesalt exchange, which is performed either prior to or aftercrystallisation, comprises treating the magnesium succinate with amonovalent base to provide a magnesium base and the monovalent succinatesalt; c) adjusting the concentration of the monovalent succinate salt inthe aqueous solution to a value between 10 and 35 wt. %; d) subjectingthe aqueous solution comprising the monovalent succinate salt towater-splitting electrodialysis, to produce a first solution comprisingmonovalent base and a second solution comprising succinic acid andmonovalent succinate salt, the electrodialysis being carried out to apartial conversion of 40 to 95 mole %; e) separating the second solutioncomprising succinic acid and monovalent succinate salt into succinicacid and a solution comprising the monovalent succinate salt bycrystallisation; f) recycling the solution of step e) comprising themonovalent succinate salt to step d).
 2. The process according to claim1, wherein in step b) the salt exchange is performed aftercrystallisation.
 3. The process according to claim 1, wherein in step c)the concentration of the monovalent succinate salt in the aqueoussolution is adjusted to a value between 20 and 35 wt. %.
 4. The processaccording to claim 1, wherein the electrodialysis step d) is carried outto a partial conversion of 50 to 95 mole %.
 5. The process according toclaim 3, wherein the electrodialysis is carried out to a partialconversion of 85 mole %.
 6. The process according to claim 1, whereinthe first solution comprising the monovalent base produced by thewater-splitting electrodialysis step d) is recycled to step b).
 7. Theprocess according to claim 1, wherein the water-splittingelectrodialysis is carried out in an electrodialysis apparatus providedwith a cation exchange membrane and a bipolar membrane.
 8. The processaccording to claim 1, wherein the magnesium base of step a) is magnesiumhydroxide.
 9. The process according to claim 1, wherein the aqueousmedium comprising the magnesium succinate is subjected to a separationstep to remove microbial cell matter prior to step b).
 10. The processaccording to claim 1, wherein the monovalent base in step b) comprises acation that is a sodium, potassium, lithium, ammonium,monoalkylammonium, dialkylammonium, trialkylammonium ortetraalkylammonium cation.
 11. The process according to claim 1, whereinthe succinic acid obtained after the separation step e) is in solid formand has a purity of at least 99 wt. %.
 12. The process according toclaim 1, wherein in step c) the concentration of the monovalentsuccinate salt in the aqueous solution is adjusted to a value between 20and 30 wt. %.
 13. The process according to claim 1, wherein in step c)the concentration of the monovalent succinate salt in the aqueoussolution is adjusted to a value between 22 and 28 wt. %.
 14. The processaccording to claim 1, wherein the electrodialysis step d) is carried outto a partial conversion of 60 to 95 mole %.
 15. The process according toclaim 1, wherein the electrodialysis step d) is carried out to a partialconversion of 70 to 90 mole %.
 16. The process according to claim 1,wherein the electrodialysis step d) is carried out to a partialconversion of 80 to 90 mole %.
 17. The process according to claim 1,wherein the succinic acid obtained after the separation step e) is insolid form and has a purity of at least 99.5 wt. %.
 18. The processaccording to claim 1, wherein the succinic acid obtained after theseparation step e) is in solid form and has a purity of at least 99.7wt. %.
 19. The process according to claim 1, wherein the succinic acidobtained after the separation step e) is in solid form and has a purityof at least 99.9 wt. %.
 20. The process according to claim 1, whereinthe monovalent base in step b) comprises a cation that is a sodium orpotassium cation.